A conventional resonance tag is composed of a capacitor and a coil, and it has been used as shown below. First, a resonance tag attached to a large number of products displayed at stores, known as a so-called shoplifting preventing tag. When a product is dishonestly taken out from a store, it passes through a portion called a gate, and then the resonance tag attached to the product receives a transmitted electromagnetic wave from the gate, thereby causing the tag to resonate to reflect an electromagnetic wave with the same frequency. This allows detecting the product being taken out dishonestly, thereby preventing shoplifting.
When a product is purchased normally, on the other hand, the resonance function of the shoplifting preventing tag is inactivated by a tag inactivator installed for example on a register. Thus the tag does not reflect electromagnetic waves, so that a product can pass the gate without any problem.
The resonance tag is inactivated by applying a high voltage generated when the resonance tag is given a pulse wave or Joule heat accompanied by the high voltage to a dielectric layer of the capacitor to cause the dielectric breakdown of the dielectric layer, thereby short-circuiting a part of the wire of the resonance circuit (refer to, for example, Japanese Patent Application Laid-Open Nos. 2002-185281, 2003-217039, and 2002-288749).
Secondly, the resonance tag is used as a recording card such as a prepaid card. As a concrete example, there is a magnetic record card in which information is recorded by the change of the resonance characteristics of the resonance circuit, as disclosed in Japanese Patent Application Laid-Open No. H11-306311. This takes advantage of resonance-characteristic changing means disposed at a resonance circuit, to be more specific, the blowout of a fuse or the dielectric breakdown of an insulating material arranged between plates of a capacitor under a condition in which some electromagnetic waves are radiated.
Japanese Patent Application Laid-Open No. 2002-245429 discloses a method of carrying out sequential dielectric breakdown of capacitors by radiating each capacitor with electromagnetic waves satisfying some condition to change the combined capacitance of the capacitors and sequentially writing information allocated to each of the resonance frequencies, in a resonance tag including a resonance circuit formed by a plurality of capacitors and a coil which are connected in series. Further, Japanese Patent Application Laid-Open No. 2003-271912 proposes a tag provided with a plurality of such resonance circuits, for representing some specific information having a relative relation of the resonance characteristics of the circuits.
However, a principle that information is written in the conventional art described above is based upon an irreversible phenomenon of the blowout of a fuse or the complete dielectric breakdown of the dielectric layers inside capacitors. For this reason, information can be recorded only once. There lacks viewpoints about usages brought about by an intentional return of a resonant function or by a reversible change in resonance frequencies, that is, those about the manufacture of a reusable shoplifting preventing tag or information-rewritable card or ID tag.